1. Field of the Invention
The present invention is in the field of semiconductor devices generally and is specifically directed to mesa type diodes and thyristors which utilize guard rings to increase their voltage capability.
2. Description of the Prior Art
With reference to FIG. 1, there is shown a prior art mesa type diode 10 with a guard ring to enable the diode to withstand high voltages.
The diode of FIG. 1 comprises an n-type cathode region 12 and a p-type anode region 14 with a p-n junction 16 therebetween. The diode 10 has a p-type guard ring region 18 spaced apart from the p-type anode region 14. There is chamfer or bevel 20 extending from top surface 22 along edge 24. The chamfer or bevel 20 has a depth "d".
Typically, in preparing the diode 10, starting with a body of n-type silicon having for example, a resistivity of about 80 ohm-cm, p-type regions 14 and 18 are formed by diffusion through surface 22. Since the regions 14 and 18 are formed during the same diffusion they will have essentially the same depth, which for a diode capable of withstanding 1600-1800 volts will be about 55.mu. to 65.mu.. The spacing W.sub.s between regions 14 and 18 along the surface 22 is about 50.mu..
The chamfer or bevel 20 is then formed by chemical etching. The chamfer or bevel typically will extend along edge 24 for a vertical distance approximately equal to twice the depth of region 18 from 110.mu. to 130.mu..
It is obvious that if the starting body of n-type silicon had an original thickness of about 145.mu. that the finished diode 10 will be relatively fragile and easily broken during handling.
This problem is even more acute when a mesa type thyristor is made with guard rings.
With reference to FIG. 2, there is shown a prior art thyristor 30.
The thyristor 30 is comprised of a cathode emitter, n-type, region 32, a p-type cathode base region 34, an n-type anode base region 36, and a p-type anode emitter region 38.
There is a p-n junction 40 between regions 32 and 34, a p-n junction 42 between regions 34 and 36 and a p-n junction 44 between regions 36 and 38. The thyristor 30 also contains a first guard ring region 40 and a second guard ring region 42.
In the usual process of making the thyristor 30, p-type regions 34, 38, 40 and 42 are formed at the same time and thus will have the same thickness or depth. Consequently, chamfers or bevels 46 and 48 each formed to twice the depth or the guard ring regions 40 and 42 respectively results in a very physically fragile body of semiconductor material which can be easily broken while handling.